Scavenger receptors (SR) are cell surface proteins which bind chemically modified lipoproteins and exhibit broad ligand binding specificities. We have identified three classes of vertebrate and invertebrate SRs: class A (SR-A), class B (SR-B) and class C (SR-C). They participate in lipoprotein metabolism, development, host defense (pathogen recognition for innate immunity, protection against septic shock and viral infection), and possibly asbestosis and the recognition and clearance of damaged (apoptotic) cells and macromolecules. Many of their functions are directly related to health and disease and are consequences of their broad ligand binding specificities. One of these, SR-BI, was the first physiologically relevant HDL receptor to be identified. It controls the levels and fates of plasma HDL cholesterol, including delivery to the liver and steroidogenic tissues. SR-BI mediates selective uptake of HDL cholesterol, a poorly understood mechanism which is distinct from classic lipoprotein endocytic uptake. The overall goals of this proposal are 1) to elucidate the biochemical and structural bases for the high affinity, broad ligand binding specificities of these receptors by determining how their ligand binding domains (e.g., collagenous and alpha-helical coiled-coil domains of SR-AI-II) recognize diverse arrays of structurally distinct ligands, 2) to provide additional insights into the novel molecular mechanism underlying selective lipid uptake, and 3) to provide both experimental tools and a biochemical framework with which to assess further the functions of these unusual receptors. The work will rely on the generation and functional analysis of mutant receptors, including domain-swap chimeras. The design of the mutants will depend, in part, on the unusual structures of these receptors, which will be explored using a variety of methods. Detailed characterization of the structures and distinctive binding properties of mammalian and invertebrate scavenger receptors will provide important tools for the analysis of scavenger receptor function and will probably suggest new approaches for the treatment and prevention of at least some of the related diseases (e.g., atherosclerosis, infectious disease). The proposed work may lead to methods for predicting which physiologically relevant molecules are receptor ligands; this would provide additional avenues for exploring receptor function and, possibly, the design of pharmacologic reagents. In addition, clarification of the molecular bases of the broad binding specificities of scavenger receptors may provide insight into other biological systems in which broad binding specificity is important, e.g., multidrug resistance.